Investigation of natural convection induced outer side heat transfer rate of coiled-tube heat exchangers

Abstract Natural convection induced heat transfer has been studied over the outer surface of helically coiled-tube heat exchangers. Several different geometrical configurations (curvature ratio δ  e [0.035, 0.082]) and a wide range of flow parameters (60  T tank T in  = 19 and 60  T in T tank  = 20, 4000  In addition to this the qualitative behavior of the peripherally averaged Nusselt number versus the axial location along the helical tube function is strongly dependent on the direction of the heat flow (from the tube to the storage tank and the reversed direction). Inner side heat transfer rate of helical coils have also been investigated in case of fluid-to-fluid boundary conditions and the calculation results have been compared with different prediction formulas published in the last couples of decades.